This invention relates to a four-wheel drive system mounted in a power train of a vehicle for changeover between transmission and non-transmission of driving force.
If a 4WD vehicle turns an a paved road with the front and rear wheels directly coupled together, a phenomenon known as tight corner braking will occur. In order to prevent such a phenomenon, rotation transmission devices having a roller type two-way clutch and an electromagnetic coil are known.
The rotation transmission device A shown in FIGS. 12 and 13 is one of them. This device is mounted in a transfer case 5 of an FR-based 4WD vehicle carrying hub clutches 2 at the front wheels 1 and having an input shaft 6 extending through the transfer case 5 from the transmission 4 coupled to the engine 3 to the rear propeller shaft 8 for the rear wheels 7 so that the entire output is directly transmitted to the rear propeller shaft 8. It comprises a roller type two-way clutch 10 for selectively coupling and uncoupling the input shaft 6 to a chain sprocket 9 coaxially and relatively rotatably mounted on the input shaft 6, and an electromagnetic clutch 11 for locking and unlocking the clutch 10. The rotation transmission device provides a 4WD AUTO (control) mode besides conventional typical part-time 4WD modes (2WD, 4WD-Hi, 4WD-Lo).
FIGS. 14A and 14B show in detail the two-way clutch 10 and the electromagnetic clutch 11 of the rotation transmission device A. The two-way clutch 10 comprises an inner member 12 mounted on the shaft 6 and an outer ring 13 mounted on the inner member 12 through bearings so as to be coaxial with and rotatable relative to the inner member 12. The inner member 12 or the outer ring 13 is formed with a plurality of cam faces 14 provided opposite a cylindrical face 15 formed in the other of the inner member 12 and the outer ring 13 to define a wedge-like space therebetween. Inserted in the wedge-like space is a retainer 16 having a plurality of pockets in which are received engaging elements in the form of rollers 17. A switch spring 18 is held by the retainer 16 and the inner member 12 or outer ring 13 that is formed with the cam faces 14 to bias the retainer 16 to a neutral position in which the rollers 17 engage neither the cylindrical face 15 nor the cam faces 14.
The electromagnetic clutch 11 comprises a friction flange 19 fixed to the outer ring 13 or the inner member 12, an armature 20 provided at one end of the retainer 16 in juxtaposition with the flange 19 with a gap therebetween so as to be slidable but nonrotatable relative to the retainer 16, and an electromagnetic coil 21 for magnetically pressing the friction flange 19 and the armature 20 against each other. By turning on or off the electromagnetic coil 21, the rollers 17 are engaged or disengaged.
This system further includes rotation sensors a and b (FIG. 12) for detecting the rotation speeds of the front and rear wheels or the front and rear propeller shafts. While a mode changeover switch 22 is at the AUTO mode position, if an ECU (controller) 23 detects that one or both rear wheels 7 are slipping based on the signals from the sensors a, b, it applies a current to the electromagnetic coil 21 in real time to lock the two-way clutch 10.
FIG. 15 shows a basic control logic in a system of this type while the vehicle is accelerating during AUTO mode. In the system, if the rotating speed of the rear wheels 7 exceeds that of the front wheels 1 by more than a predetermined value, the ECU 23 supplies a current to the electromagnetic coil 21. In FIG. 15, Vfrepresents front revolution speed, Vr represents rear revolution speed, B represents brake actuation, ABs represents ABS actuation, Vo represents set value 1 (front-rear rotation speed differences), and .increment. Vo represents set value 2 (rear wheel acceleration).
In such a system, during LOCK mode (4WD-Hi, 4WD-Low), a current is continuously supplied to the electromagnetic coil 21 to keep the two-way clutch 10 locked, thereby keeping the front and rear wheels 1, 7 directly coupled together irrespective of how the driver is operating the vehicle. Stable four-wheel drive is thus possible.
In such a conventional control system, the ECU 23 attempts to lock the two-way clutch 10 by applying a voltage to the coil 21 after a larger-than-threshold slip of a rear wheel has been detected. The two-way clutch 10 can not lock up instantly upon application of the voltage but only after a certain time period. Due to this time lag, if the vehicle is started sharply on a low-.mu. road such as a frozen road and a rear wheel spins as a result, the rotation speed difference between the front and rear vehicle wheels may grow large by the time the clutch 10 locks. Thus a large shock may be produced when the clutch 10 locks up.
Also, in a conventional system, during LOCK mode, it is necessary to keep supplying a current to the electromagnetic coil 21 even while the vehicle is at stopped with only the engine idling because the ECU cannot anticipate when the driver will start the vehicle moving. This means that a current is continuously supplied to the coil 21 even when the driver stops the car for a long time with only the engine idling or while he is out of the car. This is not only a waste of energy but can overheat the coil 21.
An object of this invention is to provide a four-wheel drive system which can reduce the shock when the two-way clutch locks up after the vehicle has started sharply on a low-.mu. road.
Another object is the provision of a four-wheel drive system that consumes less power.
The applicant of this invention proposed another rotation transmission device having rollers as engaging elements in Japanese patent publication 10-53044.
This rotation transmission device B, shown in FIG. 16, includes a two-way clutch 39 comprising an outer ring 32 having an inner cylindrical surface 34, an input shaft 33 (camshaft) inserted through the outer ring 32 and having cam surfaces 35 opposite the cylindrical surface 34 to define wedge-like spaces therebetween, a retainer 36 mounted in the space defined between the cylindrical surface 34 of the outer ring 32 and the cam surfaces 35 of the inner shaft 33, and rollers 38 received in pockets 37 formed in the retainer 36 and adapted to engage the cylindrical surface 34 and the cam surfaces 35 when the outer ring 32 and the input shaft 33 rotate relative to each other. As shown in FIG. 18, the two-way clutch 39 further includes a switch spring 40 urging the rollers 38 to a disengaged, neutral position as shown in FIG. 17. When the rollers 38 are in their neutral position, a gap x is present between each roller 38 and the outer ring 32.
Referring to FIG. 18, the switch spring 40, provided at one end of the retainer 36, has its ends engaged in cutouts 41 and 42 formed, respectively, in the input shaft 33 and the retainer 36, thereby urging the input shaft and the retainer such that their cutouts 41 and 42 align with each other. An input ring 43 is mounted through splines to the input shaft 33. (FIG. 16)
Referring back to FIG. 16, an armature 44 is axially slidably coupled to the other end of the retainer 36 by means of e.g. serrations. The rotation transmission device B further includes an electromagnetic clutch 46 having an electromagnet 45 for pressing the armature 44 against the outer ring 32.
This rotation transmission device is mounted in an FR-based 4WD vehicle on its front propeller shaft 49 connecting a transfer case 48 coupled to the engine transmission to a front differential 47 as shown in FIG. 22, or in an FF-based 4WD vehicle on its rear propeller shaft 52 connecting a center differential 50 of a viscous fluid type or gear type to a rear differential 51 as shown in FIG. 21. In either arrangement, the vehicle is equipped with an ABS, and the rotation transmission has an input ring 43 of the input shaft 33 directly connected to the input side and has its outer ring 32 directly connected to the output side.
In the abovesaid rotation transmission device, the retainer 36 is supported and guided by the outer periphery of the input shaft 33. The cutout 42 is formed in the retainer 36 at its end opposite to the end at which the armature 44 engages. As described above, the ring-shaped switch spring 40 engages this cutout 42 and the cutout 41 of the input shaft 33, thereby urging the retainer and the input shaft such that the cutouts 41 and 42 align with each other.
While the rollers 38 are in their neutral position, the force F of the switch spring 40 acts symmetrically on the retainer 36 as shown by the arrows in FIGS. 19A and 19B, keeping the cutouts 41 and 42 in alignment with each other.
When the elctromagnetic clutch 46 is energized, the armature 44 is brought into frictional contact with the outer ring 32, so that the retainer 36 rotates relative to the input shaft 33. When the retainer rotates even slightly relative to the input shaft, the force of the switch spring 40 acts on the retainer 36 at one point in one direction as shown by the arrow Fs in FIGS. 20A and 20B. At the other end, the force of the armature 44 acts on the retainer 36 in the direction opposite to the direction of the force of the switch spring applied to the retainer as shown by arrows Fc. The forces Fs and Fc cooperate to produce not only moment M1 (intended torque) about the central axis of the retainer, but also unintended moment M2 about an axis perpendicular to the central axis. The moment M1 alone acts as a force to rotate the retainer about its central axis. But the combined force of the two moments M1 and M2 tends to cause the retainer to incline or get out of alignment. This combined force is borne by the contact portion between the retainer and the input shaft 3, thus increasing the friction between the input shaft and the retainer. This makes it increasingly difficult for the retainer to rotate about its central axis relative to the input shaft.
This in turn makes it difficult to move the rollers 8 smoothly from the neutral to engaging position and vice versa.
Thus another object of this invention is to provide a rotation transmission device that ensures smooth and accurate movement of the retainer while preventing magnetic leakage.